A camshaft is used in an internal combustion engine to perform cyclical processes while the engine is in operation. For heat treatment purposes a camshaft can be geometrically described as a longitudinally oriented workpiece with at least one or more cam lobes, with each cam lobe in the shape of an eccentric cylindrical component, distributed along the central longitudinal axis of the camshaft. Generally, although not always, each cam lobe has an eccentric circular profile with the circular cam lobe center inline with the central longitudinal axis of the camshaft. In three dimensions the cam lobe may be described as an eccentric right cylindrical component aligned with the central longitudinal axis of the camshaft. There are typically multiple cam lobes distributed along the longitudinal axis of the camshaft to coordinate the opening and closing of the engine's intake and exhaust valves. Other components may form a camshaft in addition to the cam lobes. Entire camshafts may be produced by forging, casting, machining or assembly, and may be a solid, hollow or a combination solid and hollow camshaft.
Generally the spacing between adjacent cam lobes along the central longitudinal axis of a traditional camshaft is large since the components that a cam operates on (for example, an intake or exhaust valve) are spaced apart at large distances. For example a typical traditional camshaft 90 is shown in FIG. 1. The camshaft comprises four groupings (92a through 92d) of cam lobes (94a and 94b; 94c and 94d; 94e and 94f; and 94g and 94h) distributed along the axial length of cylindrical shaft 98. Additional camshaft features, such as bearings and end caps, are not shown. Typically the cam lobes are forged, cast or machined integral with the shaft and additional components, such as bearings, are added afterwards; alternatively camshaft components, for example, cam lobes, can be shrink fitted onto the camshaft. The number of cam lobes, their sizes, profiles, positioning and orientation are dependent upon the camshaft type and engine design. Camshafts, either hollow or solid, find use in many applications where one or more features on the camshaft, such as a cam lobe, must be metallurgically hardened to withstand wear and forces applied during a projected lifetime of use in an application. Typical axial widths x1 of these cam lobes on traditional automotive camshafts are on the order of 9 to 21 mm (millimeters) and typical minimum axial separation x2 between adjacent cam lobes is on the order of 12 to 40 mm.
When operating a valve, the cam lobe profile is the working surface of the cam lobe having contact with the rocker of a cam follower that is connected to the valve. During the camshaft's intended life cycle, a camshaft can rotate through millions of 360° rotational cycles and experience considerable wear and contact stresses due to sliding friction of the cam follower on the working surface of the cam lobe. The working surface for a cam lobe is illustrated with cross sectional hatching for cam lobe 94b in FIG. 1. A good combination of wear resistance and strength is essential for cam lobes, which require hardening of the working surface regions. Based on camshaft functionality, the working surface of a cam lobe comprises at least the following profile regions: the base circle (also known as the heel), the flank and the nose.
Lobe nose regions 95a and 95b, base circles 97a and 97b, and flanks 99a and 99b are shown in FIG. 2(a) and FIG. 2(b) for camshaft lobe profiles that are categorized as having either a “sharp” nose (in FIG. 2(a)) or a “regular nose” (in FIG. 2(b)) to distinguish a sharp nose region with a profile arc less than the profile arc of the typical regular nose in FIG. 2(b). The heel (base circle) is the portion of the cam lobe that is generally concentric with the shaft of the camshaft (for example, shaft 98 in FIG. 1), and has no lift of the element (such as the rocker of a cam follower) that makes contact with the working surface of the cam lobe; the flanks are the portions of the cam lobe with large acceleration and velocity to get the valve connected to the cam follower moving for opening or closing as quickly as possible; and the nose is the portion of the cam lobe with the smallest radius of curvature opposite the heel to give the greatest valve lift. Maximum cross sectional diameter of a cam lobe can be defined as the distance y1 in FIG. 2(b) from the peak of the nose to the bottom of the heel.
Various types of heating inductors (also referred to as induction coils) can be utilized to induction harden components on a workpiece of generally cylindrical shape that includes a cam lobe with an eccentric right cylindrical shape as described herein. The inductors are generally single-turn or multi-turn inductors having a circular cross sectional shape, as shown, for single turn induction coil 80 in FIG. 3. Since the intensity of induction heating is dependent upon magnetic flux coupling with regions of the workpiece to induce the eddy current heating in the workpiece or component inserted in the coil, a uniform inductive heat treatment within a complex geometry area, such as a cam lobe, is difficult to achieve with conventional induction coil arrangements. The inductive heating process is further complicated by the fact that generally heat penetration into the interior of the workpiece is a combination of both inductive eddy current heating inwardly, and then further conductive inward heat transfer (sometimes called “heat soaking”) from the eddy current regions (controlled by the depth of current penetration) towards the central region of the workpiece. Presence of workpiece regions adjacent to the workpiece region intended to be heat treated can complicate the ability of achieving required temperature uniformity in the intended heat treatment region.
Depending upon a camshaft's geometry and the required per unit heat treatment time production requirements, camshafts may be induction hardened using scan induction heating of each cam lobe with a single inductor; or static (single shot) heating of a single, or multiple lobes, with multi-turn inductors.
Scan induction hardening is typically used for lower production rates. Single turn scan inductors provide the greatest flexibility by allowing heat treating cam lobes of various widths with a minimum amount of power since only a fraction of a single cam lobe's working surface is heated in a given time period when using a scanning inductor with a narrow width face (80′ in FIG. 3) that is less than the width of the cam lobe.
Scan induction hardening can be problematic when trying to meet a specified range of “minimum-to-maximum” hardness case (surface) depth variations when heating cam lobes with an appreciably different ratio of “lobe nose diameter-to-lobe base circle diameter,” in particular when the cam lobes are positioned very close to each other. For example FIG. 4(a) and FIG. 4(b) illustrate one example of a non-traditional type of camshaft, namely a tri-lobe camshaft 70 with four tri-lobe groups (72a, 72b, 72c and 72d) each comprising three closely spaced cam lobs (lobes: 74a, 74b and 74c; 74d, 74e and 74f; 74g, 74h and 74i; and 74j, 74k and 74l) with each cam lobe having a lobe nose diameter dn to lobe base circle (heel) diameter dh ratio (dn:dh) within the range of greater than 1.5:1 or less than 1:1.5 and the axial distance x3 between a central lobe (74a, 74d, 74g or 74j) and each associated outside end lobes (74b and 74c; 74e and 74f; 74h and 74i; and 74k and 74l) in each tri-lobe group being no greater than 2 to 5 mm with distances closer to 2 mm being more typical.
A tri-lobe camshaft can be used, for example, in an engine that has free floating reciprocating pistons each with a cam follower, where the pistons are arranged in one or more banks of four pistons, each of which extends over an angular interval of 90° banks, which may be separated by an angular interval of 30°. Cam lobe profiles in these engines will approach sinusoidal shape with different profiles according to the desired characteristics of a specifics engine. A hydraulically-actuated two-piece tappets switch between profiles on the tri-lobe camshaft alternates both the lift and the duration.
An attempt to scan induction harden the closely spaced cam lobes in a tri-lobe group is inevitably associated with at least a two-fold challenge: undesirable tempering back of adjacent lobes in a tri-lobe group that were already hardened; and the possibility of obtaining spotted hardness due to quench splashes onto surfaces of cam lobes already heated in a tri-lobe group from the surface of cam lobes being quenched due to the close axial proximity of the heated cam lobes and the cam lobes being quenched particularly in a tri-lobe group.
As an alternative to scan induction hardening, a group of closely spaced cam lobes in a tri-lobe group can be induction (one shot) hardened statically by inserting the closely spaced cam lobes in a tri-lobe group, such as tri-lobe group 72a (shown as a partial camshaft section in FIG. 5(a)) within single turn inductor 80 as shown in FIG. 5(b). Single-turn inductor 80 can be a hollow copper coil inductor with internal water cooling passage 81 thus forming a single turn inductor with coil widthcoil, fixed cross sectional circular inner diameter din and outer diameter dout as shown in FIG. 5(b). A tri-lobe group of cam lobes is typically connected to at least one other tri-lobe group of cam lobes along the axial length of the camshaft as shown in FIG. 4(a) via shaft 78. Referring to the tri-lobe group in FIG. 5(a), because of the electromagnetic proximity effect, the induced heat intensity of central lobe 74a will be much lower compared to the induced heat intensities (stippled regions) of outside end lobes 74b and 74c when heat treated with a single-turn inductor as shown in FIG. 5(b); this is due to the larger gap g1 between an induction coil surface facing the central lobe (referred to as “face surface”) and the working surface of the facing central lobe than the gaps g2 between the induction coil face surface and the working surfaces of the outside end lobes within a tri-lobe group of cam lobes. Additionally in tri-lobe camshafts the outside end lobes in a tri-lobe group may be much thinner in width than the central lobe in a tri-lobe group; that is, the ratio of the width x4 of the central lobe to the ratio of the width x5 of each outside end lobe in a tri-lobe group can be greater than 2:1 as illustrated in FIG. 4(a). These factors result in appreciably deeper hardened case (surface) depth of the outside end lobes (74b and 74c) compared to the central lobe 74a as illustrated by the stippled working surface regions in outside end lobes 74b and 74c in FIG. 5(b) and practically no (stippled) working surface depth hardening of central lobe 74a. Depending upon the geometrical differences of the outside end lobes versus the central lobe in a tri-lobe group, the difference in the hardness case (surface) depth can be unacceptable. For example, in order to obtain specified minimum hardened case (surface) depth on the central lobe in a tri-lobe group, the outside end lobes in the tri-lobe group can be overheated to the extent of producing undesirable microstructures and metallurgically unacceptable results (such as, grain boundary liquation, grain coarsening, steel burning and cracking).
The use of a multi-turn inductor with each of the multiple turns having a fixed cross sectional circular inner diameter and outer diameter can also be used to heat treat cam lobes in a tri-lobe group. FIG. 6(a) illustrates in cross section cam lobes 74a, 74b and 74c in tri-lobe group 72a and three turn inductor 82 comprising middle coil turn 82b and outside end turns 82a and 82c, with the outside end coil turns axially off set (centered at axial locations Xaxis1 and Xaxis2) from outside end lobes 74b and 74c (centered at Xaxis3 and Xaxis4) to magnetically decouple the outside end coil turns from the magnetic field established when the three turn inductor is suitably connected to a power supply, and the inside cross sectional radius r1 of the middle coil turn 82b is less than the inside radius r2 of the outside end coil turns 82a and 82c. The three turn inductor arrangement in FIG. 6(a) results in an improved hardened surface (case) depth distribution (illustrated by stippled regions) for the central lobe 74a and outside end lobes 74b and 74c over that for the single turn inductor arrangement shown in FIG. 5(b) where there is no appreciable working surface hardening of central lobe 74a. Further improvement can be made to the three turn inductor arrangement in FIG. 6(a) by adding flux concentrator 62 over and around the sides of middle coil turn 82b to increase the localized magnetic flux field and the induced heat intensity in central lobe 74a as shown in FIG. 6(b) where the overall axial length and depth of the thumbnail shaped (stippled) case hardened region in the central lobe is increased over that in the FIG. 6(a) arrangement. Further improvement to the three turn inductor arrangements in FIG. 6(a) and FIG. 6(b) can be made with the four turn inductor 84 arrangement shown in FIG. 7. The four turn inductor 84 comprises two outside end coil turns 84a and 84d, and two middle coil turns 84b and 84c with all turns suitably connected to a power source. As shown by the stippled (case hardened) regions in FIG. 7 the two middle turns of coil 84 that are positioned over central lobe 74a intensify induced heating of the wide central lobe resulting in an improved hardness pattern of the central lobe and a noticeable reduction of the central lobe's undesirable thumbnail shape over that for the arrangements in FIG. 6(a) and FIG. 6(b).
It is one object of the present invention to provide a single turn inductor with improved single shot heat treatment of closely spaced multiple eccentric cylindrical components distributed along the longitudinal axis of a workpiece.
It is another object of the present invention to provide a method of single turn inductor heat treatment of closely spaced multiple eccentric cylindrical components distributed along the longitudinal axis of a workpiece.
It is another object of the present invention to provide a single turn inductor with improved single shot heat treatment of closely spaced multiple cam lobes distributed along the axial length of a camshaft.
It is another object of the present invention to provide a method of single turn inductor heat treatment of closely spaced multiple cam lobes distributed along the axial length of a camshaft.
It is another object of the present invention to provide a single turn inductor with improved single shot heat treatment of closely spaced multiple cam lobes in a tri-lobe group distributed along the axial length of a tri-lobe camshaft.
It is another object of the present invention to provide a method of single turn inductor heat treatment of closely spaced multiple cam lobes in a tri-lobe group distributed along the axial length of a tri-lobe camshaft.